A subdomain is the section of the web address which is before a domain and you've quite possibly seen a lot of subdomains while surfing around world wide web. As an illustration, many sites such as Wikipedia have versions in different languages using subdomains - en.wikipedia.org, de.wikipedia.org etc. The best thing about using a subdomain is that it can have a separate website and its own records, so you're able to even host it on a separate server. The practical use is that you could have a supplementary website, such as an e-learning portal for college students on top of the primary school site. If you use subdomains instead of subfolders, it'll be much easier to perform maintenance or to upgrade a specific site, not mentioning that it'll be more safe to have the websites separate from one another.